Candidate diazotrophs in deep-sea sediments are phylogenetically and catabolically diverse

Marine diazotrophs play a key role in determining ecosystem productivity by alleviating nitrogen limitation. However, we know little about the identity and activity of diazotrophs in deep-sea sediments (>200 m water depth), a habitat covering nearly two-thirds of the planet. Here, we identify candidate diazotrophs from Pacific Ocean sediments collected at 2,893 m water depth using 15N-DNA stable isotope probing and a novel pipeline for nifH sequence analysis. Together, these approaches indicate that the Deltaproteobacteria, predominately members of the Desulfobacterales and Desulfuromonadales, are abundant and active diazotrophs. In addition, we detect an unexpectedly diverse assemblage of low-abundance candidate diazotrophs, including members from the Acidobacteria, a phylum not previously shown to fix nitrogen. The candidate diazotrophs appear catabolically diverse, with the potential for using oxygen, nitrogen, iron, sulfur, and carbon as terminal electron acceptors. This suggests benthic diazotrophy can persist throughout a range of geochemical conditions and therefore may be a stable source of fixed nitrogen over geologic timescales. Taken together, our results reveal that nitrogen-fixing communities in deep-sea sediments are phylogenetically and catabolically diverse, and open a new line of inquiry into the ecology and biogeochemical impacts of microorganisms in deep-sea sediments.